Structure and formation method of semiconductor device structure with a dummy fin structure

ABSTRACT

Structures and formation methods of a semiconductor device structure are provided. The method includes forming a dummy fin structure over a semiconductor substrate, and forming a mask layer covering the dummy fin structure. The method further includes irradiating the mask layer, so that the mask layer is divided into an unirradiated portion and an irradiated portion, and the irradiated portion is over the dummy fin structure. The method also includes removing a top portion of the irradiated portion and a top portion of the dummy fin structure by a first etching operation, such that the dummy fin structure has a convex top surface after the first etching operation. The method includes removing a middle portion of the dummy fin structure by a second etching operation, such that the dummy fin structure has a concave top surface after the second etching operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 16/208,157, filed on Dec. 3, 2018, which is aContinuation application of U.S. patent application Ser. No. 14/815,349,filed on Jul. 31, 2015, the entire of which is incorporated by referenceherein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs. Each generation has smaller and more complexcircuits than the previous generation.

In the course of IC evolution, functional density (i.e., the number ofinterconnected devices per chip area) has generally increased whilegeometric size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling-downprocess generally provides benefits by increasing production efficiencyand lowering associated costs.

However, these advances have increased the complexity of processing andmanufacturing ICs. Since feature sizes continue to decrease, fabricationprocesses continue to become more difficult to perform. Therefore, it isa challenge to form reliable semiconductor devices at smaller andsmaller sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A-1E are cross-sectional views of various stages of a process forforming a semiconductor device structure, in accordance with someembodiments.

FIG. 2 is a cross-sectional view of a semiconductor device structure, inaccordance with some embodiments.

FIG. 3 is a cross-sectional view of a semiconductor device structure, inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matterprovided. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Some embodiments of the disclosure are described. FIGS. 1A-1E arecross-sectional views of various stages of a process for forming asemiconductor device structure, in accordance with some embodiments.Additional operations can be provided before, during, and/or after thestages described in FIGS. 1A-1E. Some of the stages that are describedcan be replaced or eliminated for different embodiments. Additionalfeatures can be added to the semiconductor device structure. Some of thefeatures described below can be replaced or eliminated for differentembodiments.

As shown in FIG. 1A, a semiconductor substrate 100 is provided. In someembodiments, the semiconductor substrate 100 is a bulk semiconductorsubstrate, such as a semiconductor wafer. For example, the semiconductorsubstrate 100 is a silicon wafer. The semiconductor substrate 100 mayinclude silicon or another elementary semiconductor material such asgermanium. In some other embodiments, the semiconductor substrate 100includes a compound semiconductor. The compound semiconductor mayinclude gallium arsenide, silicon carbide, indium arsenide, indiumphosphide, another suitable compound semiconductor, or a combinationthereof.

In some embodiments, the semiconductor substrate 100 includes asemiconductor-on-insulator (SOI) substrate. The SOI substrate may befabricated using a separation by implantation of oxygen (SIMOX) process,a wafer bonding process, another applicable method, or a combinationthereof.

As shown in FIG. 1A, a patterned hard mask 110 is formed over thesemiconductor substrate 100, in accordance with some embodiments. Thepatterned hard mask 110 has multiple openings that expose thesemiconductor substrate 100. The semiconductor substrate 100 ispartially removed through the openings of the patterned hard mask 110 toform multiple recesses (or trenches). One or more etching processes maybe used to form the recesses. As a result, multiple fin structures areformed between the recesses. As shown in FIG. 1A, some of the finstructures (such as fin structures 120) are shown. In some embodiments,the fin structures 120 substantially have the same height. In someembodiments, the height H₁ of the fin structures 120 is in a range fromabout 120 nm to about 130 nm.

In some embodiments, the patterned hard mask 110 includes a first hardmask layer and a second hard mask layer. The first hard mask layer ispositioned between the semiconductor substrate 100 and the second hardmask layer. In some embodiments, the first and second hard mask layershave different thicknesses. In some embodiments, the second hard masklayer is thicker than the first hard mask layer. In some embodiments,the first hard mask layer and the second hard mask layer are made ofsilicon nitride, silicon oxide, silicon carbide, metal nitrides such astitanium nitride and/or tantalum nitride, another suitable material, ora combination thereof. In some embodiments, the first hard mask layer ismade of silicon nitride, and the second hard mask layer is made ofsilicon oxide. One or more additional hard mask layers can be added tothe patterned hard mask 110.

For example, one or more material layers (mask layers) are depositedover the semiconductor substrate 100. Afterwards, a photoresist layer isdeposited over the material layer(s) and is patterned by one or morephotolithography processes. The patterned photoresist is used as anetching mask to pattern the material layer(s). As a result, thepatterned hard mask 110 is formed. The patterned photoresist issubsequently removed.

As shown in FIG. 1A, a regular region 130, a dense region 140 and anisolation (ISO) region 150 are defined, in accordance with someembodiments. The interval between the fin structures 120 in the regularregion 130 is greater than that in the dense region 140 and is less thanthat in the ISO region 150. In other words, the fin structures 120 inthe dense region 140 have a concentrated arrangement in comparison withthe regular region 130 and the ISO region 150. The fin structures 120 inthe ISO region 150 have a sparse arrangement in comparison with theregular region 130 and the dense region 140. In some embodiments, astatic random access memory (SRAM) or another suitable element isconfigured to be formed in the dense region 140. In some embodiments, alogic unit or another suitable element is configured to be formed in theISO region 150.

In some embodiments, one or more fin structures 120 are dummy and willbe cut in the following operations according to requirements. Therefore,fin-cut regions 160 are defined according to the fin structures 120 thatare designed to be dummy. As shown in FIG. 1A, the regular region 130,the dense region 140 and the ISO region 150 include one or more fin-cutregions 160, in accordance with some embodiments.

As shown in FIG. 1B, a mask layer 170 is deposited over thesemiconductor substrate 100, in accordance with some embodiments. Themask layer 170 covers the patterned hard mask 110 and the fin structures120. In some embodiments, the mask layer 170 is made of a photoresistmaterial. The photoresist material may include a positive photoresistmaterial or a negative photoresist material.

As shown in FIG. 1B, the mask layer 170 is irradiated (exposed) with aradiation beam through a photomask, in accordance with some embodiments.The photomask has a pattern corresponding to the fin-cut regions 160.Due to the exposure operation, the characteristics of the mask layer 170in the fin-cut regions 160 have changed. In some embodiments, a solution(such as a developing solution) is applied on the mask layer 170 suchthat the mask layer 170 in the fin-cut regions 160 has characteristicsthat have changed even further. As a result, the mask layer 170 isdivided into first portions 170A and second portions 170B.

The first portions 170A and the second portions 170B have differentphysical or chemical characteristics. The first portions 170A cover thetop surface and the sidewalls of some of the fin structures 120. Thesecond portions 170B correspond to the fin-cut regions 160 and cover thetop surface and the sidewalls of some of the fin structures 120 that aredesigned to be dummy. In some embodiments, one of the second portions170B is sandwiched between two first portions 170A. In some embodiments,one of the second portions 170B is surrounded by one of the firstportions 170A.

In some cases, an overlay (OVL) shift or a misalignment may occur duringthe exposure of the mask layer 170. As shown in FIG. 1B, one of thesecond portions 170B in the dense region 140 may fail to accuratelyalign to the corresponding fin-cut region 160. As a result, this secondportion 170B fails to cover the sidewalls of the corresponding finstructure 120 that is designed to be dummy in the dense region 140. Thecorresponding fin structure 120 in the dense region 140 has one sidewalladjoining the first portion 170A. Similarly, one of the second portions170B in the ISO region 150 may misalign to the corresponding fin-cutregion 160. Consequently, the corresponding fin structure 120 that isdesigned to be dummy in the ISO region 150 has one sidewall contactingthe first portion 170A. However, in some other embodiments, no OVL shiftor misalignment occurs during the exposure of the mask layer 170.

As shown in FIG. 1C, the upper portion of the second portions 170B andthe patterned hard mask 110 in the fin-cut regions 160 are removed, inaccordance with some embodiments. As a result, the top surface of thedummy fin structures 180 in the fin-cut regions 160 is exposed. In someembodiments, the exposed top surface of the fin structures 180 is flat.The sidewalls of the fin structures 180 are still covered or surroundedby the second portions 170B.

A first etching process is used to remove the upper portion of thesecond portions 170B and the patterned hard mask 110 in the fin-cutregions 160, in accordance with some embodiments. In some embodiments,the first etching process includes a dry etching process. In someembodiments, the dry etching process includes a plasma etching process.As mentioned above, the first portions 170A and the second portions 170Bhave different characteristics. In some embodiments, an etching rate ofthe first etching process to the second portions 170B is much higherthan an etching rate of the first etching process to the first portions170A. In some embodiments, an etching rate of the first etching processto the second portions 170B and the patterned hard mask 110 is muchhigher than an etching rate of the first etching process to the finstructures 180. Therefore, during the first etching process, the finstructures 180 are used as an etch stop layer. In some embodiments, thefin structures 180 are also partially etched by the first etchingprocess.

In some embodiments, the conditions of the first etching process, suchas the operation time, is fine-tuned such that the fin structures 180are also partially etched. Therefore, it is ensured that the finstructures 180 are exposed without being covered by the patterned hardmask 110. In some embodiments, the portions of the fin structures 180near the sidewall are etched faster than the center of the finstructures 180. As a result, the center of the etched fin structures 180has a height greater than that of the sidewalls of the etched finstructures 180. In some embodiments, one of the etched fin structures(such as a fin structure 180A) gradually becomes higher along adirection from one sidewall towards the center of the fin structure 180Aand gradually becomes higher along a direction from the other sidewalltowards the center of the fin structures 180A. In other words, the topsurface S of the fin structure 180A has a curved profile. The curvedprofile is decurved.

As mentioned above, when an OVL shift occurs during the exposure of themask layer 170, the second portions 170B fail to cover the sidewalls ofthe corresponding fin structures 180. A portion of the patterned hardmask 110 may be embedded in the first portion 170A. As a result, theportion of the patterned hard mask 110 remains after the first etchingprocess. When the fin structures 180 are partially etched during thefirst etching process, the remaining portion of the patterned hard mask110 prevents a portion of the fin structures 180 underlying theremaining portion from being removed. Consequently, a fin structure(such as a fin structure 180B₁) with a protruding portion P is formed.The center of the fin structure 180B₁ has a height greater than that ofone sidewall of the fin structure 180B₁. In some embodiments, the finstructure 180B₁ gradually becomes higher along a direction from onesidewall through the center of the fin structures 180B₁ towards theprotruding portion P.

Loading effects may occur in the dense region 140 during the firstetching process as a result of a highly concentrated arrangement.Consequently, in some embodiments, the fin structures 180 in the denseregion 140 are slightly higher than that in the regular region 130 andthe ISO region 150. In some embodiments, one of the fin structures 180(such as a fin structure 180B₂) in the dense region 140 has a slightlylonger protruding portion P than that in the regular region 130 and theISO region 150.

As shown in FIG. 1D, a portion of the fin structures 180 in the fin-cutregions 160 are further removed to form recessed fin structures 180′, inaccordance with some embodiments. In some embodiments, a second etchingprocess is used to partially remove the fin structures 180 in thefin-cut regions 160. In some embodiments, the second etching processincludes a dry etching process. The dry etching process may include aplasma etching process. In some embodiments, the operation power usedfor performing the second etching process is less than that used forperforming the first etching process.

The second etching process uses an etchant having high etchingselectivity to the fin structures 180, in accordance with someembodiments. In other words, an etching rate of the second etchingprocess to the fin structures 180 is much higher than an etching rate ofthe second etching process to the mask layer 170. Therefore, the secondetching process is a self-aligned etching process. In some embodiments,no photolithography process is performed between the first and secondetching processes. An etching rate of the second etching operation tothe fin structures 180 is higher than an etching rate of the firstetching operation to the fin structures 180. In some embodiments, anetching rate of the second etching process to the second portions 170Bis much higher than an etching rate of the second etching process to thefirst portions 170A.

In some embodiments, the etchant having high etching selectivity to thefin structures 180 includes HBr and Cl₂ or another suitable etching gas.The fin structures 180 are etched mainly by being reacted with theetching gas. The conditions of the second etching process, such as theflow rate of the etching gas, is fine-tuned to reduce the height of thefin structures 180. Loading effect can be reduced or eliminated duringthe second etching process. As a result, the recessed fin structures180′ in the regular region 130, the dense region 140 and the ISO region150 have an uniform height, in accordance with some embodiments. In someother embodiments, the fin structures 180′ in the ISO region 150 areslightly higher than the fin structures 180 in the regular region 130and the dense region 140.

The center of the fin structures 180 is etched faster than the sidewallsof the fin structures 180, in accordance with some embodiments. As aresult, the sidewalls of the recessed fin structures 180′ surround arecess (or trench) over the fin structures 180′. The recesses will bedescribed in more detail according to FIGS. 2 and 3.

The profile of the top surface of the dummy fin structures 180 ischanged due to the second etching process, in accordance with someembodiments. Before the second etching process, the top surface of thedummy fin structures (the fin structures 180) is flat or has a decurvedprofile. After the second etching process, the top surface of the dummyfin structures (the fin structures 180′) has a sunken profile as aresult of the formation of the recess. In some embodiments, the decurvedtop surface S of the fin structures 180B₁ shown in FIG. 1C becomessunken after the second etching process. In some embodiments, theprotruding portion P of the fin structures 180B₁ and 180B₂ shown in FIG.1C is removed after the second etching process.

Afterwards, the mask layer 170 is removed from the semiconductorsubstrate 100. In some embodiments, a remaining portion of the patternedhard mask 110 after the first etching process is removed together withthe mask layer 170.

As shown in FIG. 1E, an isolation feature 190 is formed over thesemiconductor substrate 100, in accordance with some embodiments. Insome embodiments, the isolation feature 190 includes a shallow trenchisolation (STI) feature, another suitable isolation feature, or acombination thereof.

In some embodiments, the isolation feature 190 has a multi-layerstructure. In some embodiments, the isolation feature 190 is made of adielectric material. The dielectric material includes silicon oxide,silicon nitride, silicon oxynitride, fluoride-doped silicate glass(FSG), low-K dielectric material, another suitable material, or acombination thereof. In some embodiments, an STI liner (not shown) isformed to reduce crystalline defects at the interface between thesemiconductor substrate 100 and the isolation feature 190. Similarly,the STI liner may also be used to reduce crystalline defects at theinterface between the fin structures 120 and 180′ and the isolationfeature 190.

In some embodiments, a dielectric material layer is deposited over thesemiconductor substrate 100. The dielectric material layer covers thefin structures 120 and 180′ and fills the recesses between the finstructures 120 and 180′. In some embodiments, the dielectric materiallayer is deposited using a chemical vapor deposition (CVD) process, aspin-on process, another applicable process, or a combination thereof.In some embodiments, a planarization process is performed to thin downthe dielectric material layer until the patterned hard mask 110 over thefin structures 120 is exposed. In some embodiments, a planarizationprocess is performed to thin down the dielectric material layer andremove the patterned hard mask 110 until the fin structures 120 areexposed. The planarization process may include a chemical mechanicalpolishing (CMP) process, a grinding process, an etching process, anotherapplicable process, or a combination thereof.

Afterwards, the dielectric material layer is etched back to below thetop of the fin structures 120. The fin structures 180′ are still wrappedin the dielectric material layer. As a result, the isolation feature 190is formed. In some embodiments, the lower portion of the fin structures120 is surrounded by the isolation feature 190. The upper portion of thefin structures 120 protrudes from the isolation feature 190. In someembodiments, the top surface of the fin structures 180′ is covered bythe isolation feature 190. The fin structures 180′ are embedded in theisolation feature 190. In some embodiments, a height difference H₂between the isolation feature 190 and the fin structures 180′ is in arange from about 20 nm to about 40 nm.

The partially removed fin structures 180′ are used as a buffer feature,in accordance with some embodiments. The buffer feature mitigates stressaccumulated in the isolation feature 190. As a result, the finstructures 120 partially buried in the isolation feature 190 areprevented from being damaged. In some embodiments, the fin structures180′ in the regular region 130, the dense region 140 and the ISO region150 have an uniform height. Consequently, a better uniformity isachieved during the deposition of the isolation feature 190.

The patterned hard mask 110 over the fin structures 120 is removed afterthe formation of the isolation feature 190, in accordance with someembodiments. Various features will be formed over the semiconductorsubstrate 100. Some of the various features are electrically connectedto the fin structures 120. The various features may include gatestructures, contacts and another suitable feature.

The fin structures 180′ are immersed in the isolation feature 190.Therefore, the subsequently formed feature is physically separated fromthe fin structures 180′ by the isolation feature 190. In someembodiments, the fin structures 180′ have no contact with subsequentlyformed conductive features, such as contacts. As a result, there is noelectrical contact or connection between the fin structures 180′ and thesubsequently formed conductive feature. In some embodiments, a distancebetween the subsequently formed conductive feature and the finstructures 180′ is in a range from about 20 nm to about 40 nm. A shortcircuiting is prevented between the fin structures 180′ and thesubsequently formed conductive feature. Therefore, the performance andreliability of the semiconductor device structure are significantlyimproved.

FIG. 2 is an enlarged cross-sectional view of a semiconductor devicestructure, in accordance with some embodiments. In some embodiments,FIG. 2 shows an enlarged cross-sectional view of the region A shown inFIG. 1E. Two of the fin structures 180′ (fin structures 180′A and 180′B)in the ISO region 150 are shown in FIG. 2. The fin structure 180′A has afirst sidewall 210, a second sidewall 220 and a top surface 230. Thefirst sidewall 210 is opposite to the second sidewall 220. The topsurface 230 has a center 240, a first edge 250 and a second edge 260.The first edge 250 is the top of the first sidewall 210. The second edge260 is the top of the second sidewall 220. In some embodiments, thedistance between the center 240 and the first edge 250 is substantiallythe same as that between the center 240 and the second edge 260. In someother embodiments, the distance between the center 240 and the firstedge 250 is different from that between the center 240 and the secondedge 260.

In some embodiments, the first sidewall 210 has the same height as thatof the second sidewall 220. Each of the heights of the first sidewall210 and the second sidewall 220 is greater than that of the center 240.In some embodiments, the first sidewall 210 and the second sidewall 220surround a recess 270 over the fin structure 180′A. The bottom surfaceof the recess 270 is the top surface 230. In some embodiments, therecess 270 has a curved profile. In some embodiments, the recess 270 hasa symmetrical profile. In some other embodiments, the recess 270 has anasymmetrical profile.

In some embodiments, the fin structure 180′A gradually becomes shorteralong a direction from the first sidewall 210 towards the center 240 andalong a direction from the second sidewall 220 towards the center 240.In some embodiments, the fin structure 180′A gradually becomes shorteralong a direction from the first edge 250 towards the center 240 andalong a direction from the second edge 260 towards the center 240. Thefin structure 180′A may gradually become shorter along a direction fromthe first sidewall 210 or the second sidewall 220 towards the center240. In some embodiments, a height difference between the center 240 andthe first sidewall 210 is in a range from about 1 nm to about 3 nm. Insome embodiments, a height difference between the center 240 and thesecond sidewall 220 is in a range from about 1 nm to about 3 nm.

In some embodiments, a first distance 280 between the center 240 and thetop surface 200 of the isolation feature 190 is greater than a seconddistance 290 between the first edge 250 and the top surface 200. In someembodiments, the first distance 280 is greater than a third distance 300between the second edge 260 and the top surface 200. In someembodiments, the second distance 290 is substantially the same as thethird distance 300. In some other embodiments, the second distance 290and the third distance 300 are different from each other. In someembodiments, the second distance 290 and the third distance 300 are in arange from about 20 nm to about 40 nm. In some embodiments, thedifference between the first distance 280 and the second distance 290 isin a range from about 1 nm to about 3 nm. The difference between thefirst distance 280 and the third distance 300 is in a range from about 1nm to about 3 nm.

Many variations and/or modifications can be made to embodiments of thedisclosure. In some embodiments, the first sidewall 210 and the secondsidewall 220 of the fin structure 180′ (such as the fin structure 180′B)have different heights. The heights of the first sidewall 210 and thesecond sidewall 220 may be different as a result of an OVL shift. Asshown in FIG. 2, the heights of the first sidewall 210 and the secondsidewall 220 of the fin structure 180′B are different. In someembodiments, the first sidewall 210 is higher than the second sidewall220. The heights of the first sidewall 210 and the second sidewall 220are greater than that of the center 240. Therefore, the first sidewall210 and the second sidewall 220 surround a recess 270 over the finstructure 180′B. In some embodiments, the recess 270 has a curvedprofile. The recess 270 has an asymmetrical profile.

In some embodiments, the fin structure 180′B gradually becomes shorteralong a direction from the first sidewall 210 towards the center 240 andalong a direction from the second sidewall 220 towards the center 240.In some embodiments, the fin structure 180′B gradually becomes shorteralong a direction from the first edge 250 towards the center 240 andalong a direction from the second edge 260 towards the center 240. Thefin structure 180′B may gradually become shorter along a direction fromthe first edge 250 or the second edge 260 towards the center 240.

FIG. 3 is an enlarged cross-sectional view of a semiconductor devicestructure, in accordance with some embodiments. In some embodiments,FIG. 3 shows an enlarged cross-sectional view of the region B shown inFIG. 1E. A fin structure 180′C in the dense region 140 is shown. Loadingeffects may occur in the dense region 140 during operations. As aresult, a height difference between the center 240 and the firstsidewall 210 or the second sidewall 220 of the fin structure 180′C isless than that of the fin structure 180′A in the ISO region 150. In someembodiments, the fin structure 180′C is slightly higher than the finstructure 180′A and the fin structure 180′B in the ISO region 150. Insome embodiments, the recess 270 in the dense region 140 has a dimensionless than that of the recess 270 in the ISO region 150.

In some embodiments, the heights of the first sidewall 210 and thesecond sidewall 220 of the fin structure 180′ in the dense region 140are different as a result of an OVL shift. The recess 270 over the finstructure 180′ in the dense region 140 may have an asymmetrical profile.

Embodiments of the disclosure form a semiconductor device structure witha partially removed fin structure using a first etching operation and asecond etching operation. During the first etching operation, the finstructure is used as an etch stop layer. The second etching operationhas a relatively high etching rate to the fin structure in comparisonwith the first etching operation. As a result, the partially removed finstructure is low enough to be completely embedded in the isolationfeature. A short circuiting is prevented between the fin structure andconductive features subsequently formed over the isolation feature. Evenif an OVL shift and/or loading effect occur before the second etchingprocess, the induced defects are eliminated by the second etchingprocess. Therefore, the performance and reliability of the semiconductordevice structure are greatly improved.

In accordance with some embodiments, a method for forming asemiconductor device structure is provided. The method includes forminga dummy fin structure over a semiconductor substrate, and forming a masklayer covering the dummy fin structure. The method further includesirradiating the mask layer, so that the mask layer is divided into anunirradiated portion and an irradiated portion, and the irradiatedportion is over the dummy fin structure. The method also includesremoving a top portion of the irradiated portion and a top portion ofthe dummy fin structure by a first etching operation, such that thedummy fin structure has a convex top surface after the first etchingoperation. The method includes removing a middle portion of the dummyfin structure by a second etching operation, such that the dummy finstructure has a concave top surface after the second etching operation.

In accordance with some embodiments, a method for forming asemiconductor device structure is provided. The method includes forminga fin structure and a dummy fin structure, and forming a mask layercovering the fin structure and the dummy fin structure. The method alsoincludes etching a portion of the mask layer by a first etchingoperation using the dummy fin structure as an etch stop layer, andetching a portion of the dummy fin structure by a second etchingoperation to form a recess over a top surface of the dummy finstructure. The recess has an asymmetrical profile.

In accordance with some embodiments, a method for forming asemiconductor device structure is provided. The method includes forminga fin structure and a dummy fin structure, and forming a mask layer overthe fin structure and the dummy fin structure. The method also includesetching a portion of the mask layer to form an etched mask layer, andremoving a portion of the dummy fin structure, such that a top surfaceof the dummy fin structure is lower than a top surface of the etchedmask layer. The method includes removing the etched mask layer, andforming an isolation structure over the fin structure and the dummy finstructure. The dummy fin structure is embedded in the isolationstructure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for forming a semiconductor devicestructure, comprising: forming a dummy fin structure over asemiconductor substrate; forming a mask layer covering the dummy finstructure; irradiating the mask layer, so that the mask layer is dividedinto an unirradiated portion and an irradiated portion, wherein theirradiated portion is over the dummy fin structure; removing a topportion of the irradiated portion and a top portion of the dummy finstructure by a first etching operation, such that the dummy finstructure has a convex top surface after the first etching operation;and removing a middle portion of the dummy fin structure by a secondetching operation, such that the dummy fin structure has a concave topsurface after the second etching operation.
 2. The method for forming asemiconductor device structure as claimed in claim 1, wherein a firstsidewall of the dummy fin structure is covered by the unirradiatedportion, and a second sidewall of the dummy fin structure is covered bythe irradiated portion.
 3. The method for forming a semiconductor devicestructure as claimed in claim 1, further comprising: removing the masklayer after the second etching operation; and forming an isolationfeature over the semiconductor substrate to cover a top surface of thedummy fin structure after the mask layer is removed, wherein the dummyfin structure is embedded in the isolation structure.
 4. The method forforming a semiconductor device structure as claimed in claim 3, whereinthe dummy fin has a first sidewall and a center, a first distance isbetween center and a top surface of the isolation feature, a seconddistance is between a top of the first sidewall and the top surface ofthe isolation feature, and the first distance is greater than the seconddistance.
 5. The method for forming a semiconductor device structure asclaimed in claim 4, wherein a height of the first sidewall of the dummyfin structure is greater than a height of the second sidewall of thedummy fin structure after the second etching operation.
 6. The methodfor forming a semiconductor device structure as claimed in claim 1,further comprising: forming a fin structure over the semiconductorsubstrate; and forming the mask layer over the fin structure, whereinthe unirradiated portion is on the fin structure.
 7. The method forforming a semiconductor device structure as claimed in claim 6, whereinthe fin structure remains covered by the mask layer during and after thesecond etching operation.
 8. The method for forming a semiconductordevice structure as claimed in claim 1, wherein the convex top surfaceof the dummy fin has an asymmetrical profile.
 9. The method for forminga semiconductor device structure as claimed in claim 1, furthercomprising: forming a hard mask over the fin structure and the dummy finstructure, and the hard mask is covered by the mask layer, wherein aportion of the hard mask over the second fin structure is removed duringthe first etching operation.
 10. A method for forming a semiconductordevice structure, comprising: forming a fin structure and a dummy finstructure; forming a mask layer covering the fin structure and the dummyfin structure; removing a portion of the mask layer, wherein the portionof the mask layer covers the dummy fin structure; and etching a portionof the dummy fin structure by a second etching operation to form arecess over a top surface of the dummy fin structure, wherein the recesshas an asymmetrical profile.
 11. The method for forming a semiconductordevice structure as claimed in claim 10, further comprising: removingthe mask layer after the second etching operation; and forming anisolation feature over the semiconductor substrate, wherein the recessis embedded in the isolation feature.
 12. The method for forming asemiconductor device structure as claimed in claim 10, furthercomprising: irradiating the mask layer so that the mask layer is dividedinto an unirradiated portion and an irradiated portion before the firstetching operation, wherein a first sidewall of the dummy fin structureis covered by the unirradiated portion, and a second sidewall of thedummy fin structure is covered by the irradiated portion.
 13. The methodfor forming a semiconductor device structure as claimed in claim 10,wherein the fin structure remains covered by the mask layer during andafter the second etching operation.
 14. The method for forming asemiconductor device structure as claimed in claim 10, furthercomprising: forming a hard mask over the fin structure and the dummy finstructure, and the hard mask is covered by the mask layer, wherein aportion of the hard mask over the dummy fin structure is removed duringthe first etching operation, and the fin structure remain covered byanother portion of the hard mask during and after the first etchingoperation.
 15. The method for forming a semiconductor device structureas claimed in claim 10, wherein after etching the portion of the dummyfin structure, the dummy fin structure comprises a first sidewall, asecond sidewall opposite to the first sidewall, and a center between thefirst sidewall and the second sidewall, and wherein the dummy finstructure gradually becomes shorter from the first sidewall and thesecond sidewall to the center.
 16. The method for forming asemiconductor device structure as claimed in claim 10, furthercomprising: removing the mask layer after the second etching operation,wherein a top surface of the dummy fin structure is higher than a bottomsurface of the fin structure after removing the mask layer.
 17. A methodfor forming a semiconductor device structure, comprising: forming a finstructure and a dummy fin structure; forming a mask layer over the finstructure and the dummy fin structure; etching a portion of the masklayer to form an etched mask layer; removing a portion of the dummy finstructure, such that a top surface of the dummy fin structure is lowerthan a top surface of the etched mask layer; removing the etched masklayer; and forming an isolation structure over the fin structure and thedummy fin structure, wherein the dummy fin structure is embedded in theisolation structure.
 18. The method for forming a semiconductor devicestructure as claimed in claim 17, further comprising: irradiating themask layer so that the mask layer is divided into an unirradiatedportion and an irradiated portion, a remaining portion of the irradiatedportion is the etched mask layer after etching the portion of the masklayer, wherein a first sidewall of the dummy fin structure is in directcontact with the unirradiated portion, and a second sidewall of thedummy fin structure is in direct contact with the etched mask layer. 19.The method for forming a semiconductor device structure as claimed inclaim 17, wherein a top surface of the dummy fin structure is higherthan a bottom surface of the fin structure after removing the etchedmask layer.
 20. The method for forming a semiconductor device structureas claimed in claim 17, wherein after removing the portion of the dummyfin structure, the dummy fin structure comprises a first sidewall, asecond sidewall opposite to the first sidewall, and a center between thefirst sidewall and the second sidewall, and wherein the dummy finstructure gradually becomes shorter from the first sidewall and thesecond sidewall to the center.